Koichi Haneda

Morphology and magnetic properties of ultrafine ZnFe2O4 particles

Well-crystallized ultrafine ZnFe2O4 particles of several nanometers in size have been prepared by the coprecipitation method, and their particle morphology and magnetic properties, especially at low temperatures, examined. Room-temperature X-ray diffraction, transmission electron microscopy, magnetization measurements at various temperatures from 300 K to 4.2 K, and Mössbauer spectroscopy at various temperatures from 300 K to 4.2 K, and at 4.2 K with a longitudinal magnetic field of 16.4 kOe applied have been employed. The formation of short-range and long-range magnetic order in small ZnFe2O4 particles above and below approximately 30 K is discussed. Below 30 K, the appearance of spontaneous magnetization and its hysteretic property is confirmed for small ZnFe2O4 particles.

Magnetization reversal process in chemically precipitated and ordinary prepared BaFe12O19

BaFe12O19 particles were prepared by chemical precipitation and ordinary powder metallurgy methods. These particles were oriented in paraffin wax and the angular variation of coercive force was measured. By using the modified equation of Buessem et al., its variation was analyzed. It was found that reverse‐domain nucleation greatly influences the intrinsic coercive force of BaFe12O19 particles; even stress‐free chemically precipitated single‐domain particles were influenced. Annealing the ball milled powder raised the coercive force drastically. This is due to the reduction of defect‐nucleation centers. It should be noted that by the chemical precipitation method a coercive force of 6000 Oe was obtained, which is one of the highest reported up to this time for isotropic BaFe12O19.

Intrinsic Coercivity of Substituted BaFe12O19

Intrinsic coercive force measurements were made on BaFe12O19 in which certain amounts of Fe were substituted by In, Cr, Al, Ga, ZnGe, ZnV, ZnNb or ZnTa. The specimens were prepared by the ordinary powder metallurgy method. X-ray diffraction of the specimens indicated only the presence of the magnetoplumbite phase within the experimental substitution range. A large increase in coercive force iHc, observed when Cr, Al or Ga were substituted, was mainly due to the onset or the approach of single-domain behaviour at higher concentrations even under the same milling condition. In the case of Al substitution, the increase of magnetocrystalline anisotropy field Ha also contribute greatly to the abrupt rise of iHc. In the case of the other substitutions, the value of iHc decreased without an appreciable change in the single-domain critical size; this decrease might be due to the decrease in Ha.

Preparation of small particles of SrFe 12 O 19 with high coercivity by hydrolysis of metal-organic complexes

Small particles (50-60nm) of Sr ferrite (SrFe 12 O 19 ) have been synthesized by the hydrolysis method of metal-organic complexes such as metal acetylacetonates or metal ethoxides and subsequent heat-treatments of the precipitates, and their magnetic properties have been studied. X-ray, Mossbauer and electron-microscope studies have been conducted to examine the mechanism of the formation. A coercive force of 6700 Oe, one of the highest values reported so far for isotropic SrFe 12 O 19 samples and close to the theoretical limit, has been obtained. Some of their magnetic properties have been discussed in terms of magnetism of small particles.

Noncollinearity as a size effect of CrO2 small particles

The magnetic structure of small CrO2 particles doped with about 1.2 wt.% 57Fe has been explored by 57Fe Mossbauer spectroscopy as a function of the particle size. Mossbauer spectra, obtained with a longitudinal magnetic field applied, unambiguously establish that a noncollinear structure exists that is most pronounced for the smallest particles. The analysis indicates that a surface effect is the origin of this phenomenon. It follows that a core model proposed earlier for the magnetic structure of CrO2 small particles is warranted.

Magnetic excitations in small NiFe2O4 particles

Abstract Mossbauer absorption spectra for small NiFe 2 O 4 particles with and without applying a high magnetic field have been discussed in connection with the apparently anomalous ratio of the B to A-site pattern for a completely inverse spinel. Analyses indicate that the collective magnetic excitation mechanism is acting.

Effect of crystalline properties on coercive force in iron acicular fine particles

Crystallographic properties and their size dependence in acicular fine particles of iron prepared by reduction fromα-FeOOH were investigated by transmission electron microscopy and compared with magnetic properties. Although both larger particles with an average needle length of 0.5μm and smaller ones of 0.2μm length have single domain configuration, the former are crystal lographicaIly polycrystalline while the latter are single crystal with a [100] axial orientation, an easy magnetization direction of iron. The coercive force, principally yielded by external shape anisotropy, is higher in smaller particles in spite of their inferior axial ratio. The intrinsic magnetocrystalline anisotropy is inferred to be the most responsible to give rise to this further increase of coercive force. The experimentally obtained values of magnetic properties are also compared with the theoretical estimation, which results semi-quantitatively in good accordance.

Synthesis of iron—cobalt ultrafine particles by decomposition of Fe(CO)5Co(CO)3NO using a TEA CO2 laser

Abstract Ultrafine particles of iron—cobalt alloy were synthesized and isolated at room temperature in a maximum yield of 90% by the dielectric breakdown of a mixture of Fe(CO) 5 (5 Torr) and Co(CO) 3 NO (0.5 Torr) on irradiation with a tightly-focused laser beam of a transversely-excited-atmospheric (TEA) CO 2 laser. CO and NO are also obtained as volatile products. Transmission electron micrographs of the iron-cobalt particles show the existence of fine and well-organized spherical particles. The distribution of the particle size is relatively uniform in the range 6–9 nm with a mean particle size of 8 nm. In X-ray diffractometry, the ultrafine particles are found to be mainly f.c.c. γ-iron—cobalt alloy which is normally stable in the range 1183–1234 K to 1673–1773 K. The b.c.c. α-iron—cobalt, iron oxides and cobalt oxides are observed as minor products (γ-iron—cobalt to ironcobalt oxides=8:1:1). The ratio of iron and cobalt atoms in the particles can be changed by changing the mixture of Fe(CO) 5 and Co(CO) 3 NO. The ratio of γ-iron—cobalt to α-iron—cobalt to iron—cobalt oxide also depends on the ratio of iron and cobalt atoms in the particles.

Improvement of temperature dependence of remanence in ferrite permanent magnets

A low temperature coefficient of remanence has been found for ferrite permanent magnets in which some of the Fe3+ ions are replaced by non‐magnetic ions in the following ways: 2Fe3+→M2+ + M4+ or 3Fe3+→2M2+ + M5+. By replacing Fe3+ by (Cu2+, Ge4+), (Cu2+, Si4+), (Cu2+, V5+), (Cu2+, Nb5+), or (Cu2+, Ta5+), the temperature coefficient of Br between room temperature and 100°C was reduced from −0.2%/°C for BaFe12O19 to about −0.1%/°C. Substitution of (Cu, Nb) is especially effective, and gives practically no change in Br between room temperature and 50°C. These results cannot be simply explained by a decrease in the temperature dependence of the saturation magnetization. Although further research is required to clarify the mechanism, the increase in BHc with temperature may provide a phenomenological explanation.